1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and an apparatus of estimating a frequency offset based on a partial periodogram in a wireless communication system.
2. Related Art
In the case of a broadband wireless communication system, effective transmission/reception techniques and utilization schemes have been proposed in order to maximize efficiency of limited wireless resources. Next-generation wireless communication is an orthogonal frequency division multiplexing (OFDM) system capable of reducing an inter-symbol interference effect. In the OFDM, a data symbol input in series is converted into N parallel data symbols, which are transmitted with the N parallel data symbols being loaded on N divided subcarriers, respectively. The subcarriers maintain orthogonality in terms of a frequency. Respective orthogonal channels go through inter-dependent frequency selective fading, and as a result, complexity is reduced at a receiver and an interval of the transmitted symbols is lengthened, thereby minimizing inter-symbol interference. However, the OFDM system is very sensitive to the frequency offset. The frequency offset may occur by inconsistency in frequency of an oscillator between a transmitter and a receiver or a Doppler effect. The orthogonality among the subcarriers may be broken and interference may occur due to the frequency offset, and as a result, demodulation performance is reduced. This problem is commonly raised in all communication systems using the OFDM technique, which includes an OFDM-based cognitive radio (CR) system.
In order to estimate the frequency offset of the OFDM system, various frequency offset estimating methods have been proposed. First, a frequency offset estimating method based on two repeated OFDM signals and a maximum likelihood theory may be proposed. However, this method has a disadvantage that an estimate range of the frequency offset is too small. T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun, vol. 45, no. 12, pp. 1613-1621, December 1997 proposes a method of estimating the frequency offset by using a training symbol having a repeated structure in one OFDM signal and a training symbol configured by a pseudo noise code. The method of estimating the frequency offset by using the training symbol is excellent in estimation performance of the frequency offset, but has a disadvantage that the frequency offset can be estimated only when the OFDM signal is configured by a specified training symbol. J.-J. van de Beek, M. Sandell, and P. O. Borjesson, “ML estimation of time and frequency offset in OFDM systems,” IEEE Trans. Sig. Process., vol. 45, no. 7, pp. 1800-1805, July 1997 proposes a blind-based frequency offset estimating method, but is not excellent in estimation performance of the frequency offset.
As a result, a method of estimating the frequency offset based on a periodogram may be proposed. The method of estimating the frequency offset based on the periodogram provides a frequency offset estimating method which can be applied to a predetermined training symbol based on the envelope equalized processing (EEP) and the periodogram.
However, the existing proposed method of estimating the frequency offset based on the periodogram is too high in amount of computation. As a result, a frequency offset estimating method capable of lowering the amount of computation and complexity while using the periodogram needs to be proposed.